Swift Functional Programming - Second Edition

Partitioning an array

We can use reduce to partition an array with a specific criterion. For instance, in the following example, we partition numbersToPartitionArray into two partitions, keeping all even numbers in the left partition:

func partition<T>(list: [T], criteria: (T) -> Bool) -> (lPartition: [T], rPartition: [T]) {     return list.reduce((lPartition: [T](), rPartition: [T]())) {         (accumlator: (lPartition: [T], rPartition: [T]), pivot: T) ->           (lPartition: [T], rPartition: [T]) in         if criteria(pivot) {             return (lPartition: accumlator.lPartition + [pivot],                    rPartition: accumlator.rPartition)         } else {             return (rPartition: accumlator.rPartition + [pivot],                    lPartition: accumlator.lPartition)         }     } } let doublesToPartition = [3.0, 4.0, ...